The present state-of-the-art in coating wires or other materials with polyurethanes generally involves the use of one of several methods. The most commonly employed methods involve the use of extrusion or coextrusion techniques. In the extrusion technique, a tube is made for the wire to be coated through which the wire can later be slipped. In coextrusion, the urethane material is applied directly to the wire being covered. Both of these methods involve the use of heat to melt the polymer and high pressure to push the rather highly viscous material out of the extrusion orifice. These processes have limitations in that the thickness of the urethane coating being applied must be relatively great to achieve uniformity and a smooth defect-free finish. Therefore, such techniques are not very useful when one wants to apply a coating only a few one-thousandths of an inch thick since close tolerance between orifice and wire diameters is required.
The urethanes are also adaptable to vacuum and injection molding systems. Again, like extrusion, high temperature melting is required and the adaptability of such techniques for small wire coating is severely limited.
Another technique called "lacquering" is used much less commonly for coating wire or other materials. In this technique, the urethane is dissolved in a volatile solvent and then the resultant solution is sprayed on the object to be coated. Alternately, the object is dipped into the solution for coating much as one would make candles. The solvent is then evaporated at temperatures up to 200.degree. C. leaving an adherent film of urethane to harden on the surface. This latter technique is disadvantaged since the process is slow and provides only limited production possibilities. In addition because the urethane lacquers are quite thin when applied they tend to run off or sag on the surface to be covered.